1. Field of the Invention
The present invention concerns a method for triggered implementation of a measurement composed of partial measurements in a magnetic resonance tomography apparatus, of the type wherein at least one image data set is determined from the data acquired within the scope of the partial measurements, and wherein a reference point of the movement phase of the movement is used for triggering.
2. Description of the Prior Art
For data acquisition of magnetic resonance tomography data, it is known to acquire the data in an EKG-triggered manner in order to minimize movement artifacts from the post-processed image data. The EKG signal (wherein EKG is the generally known abbreviation for electrocardiogram) is a measure of the electrical activity of the heart muscle fibers. The contraction of the heart muscle is produced by an electrical excitation, which is detected with the EKG apparatus. The EKG signal is a periodic signal and the different phases of the cardiac cycle are designated with letters. The conventional sequence is P-Q-R-S-T-U. The R-spike exhibits the greatest amplitude; it is therefore normally used as a reference point of the triggering.
The detection of the EKG signal ensues via electrodes attached to the patient. The signals detected by the electrodes are relayed via electrical lines from the magnetic resonance tomography apparatus to an EKG device. Naturally the EKG signal can be filtered or post-processed in another manner.
Furthermore, it is known to trigger measurements depending on the breathing movement. For this purpose, a sensor is provided that transduces the breathing movement into an electrical signal. This signal is a measure of the breathing movements and thus can be used to control the magnetic resonance device. Only the EKG triggering, however, is discussed in detail in the following.
The term “data acquisition” is used herein to encompass the entire measurement procedure. In contrast to the data acquisition, the time period during which actual data are stored in a storage medium is designated as measurement with data acquisition. During the magnetic resonance data acquisition, the readout gradient is typically activated for frequency coding in order to provide the measurement signal with a spatial coding during the data acquisition.
A number of differently designed magnetic resonance measurements or measurement methods are available. The terms “data acquisition” or “measurements” encompass all types of measurements. One or more two-dimensional or three-dimensional image data sets, multiple image data sets from different slices of the examination subject and data sets with and without weighting with regard to a specific parameter can be obtained as a result of the post-processing of the acquired raw data. Within the scope of the post-processing it can also be necessary to supplement the data acquired within the scope of one measurement with data of other measurements, or to have to draw on additional data from other measurements for post-processing in order to obtain one or more image data sets from the acquired data.
Such a measurement is composed of partial measurements. The partial measurements are basically of the same design; only the strength of the phase-coding gradient or of the slice gradients (and rarely also of the readout gradient) is varied in a predetermined manner. The partial measurement can include one or more preparation modules in order to weight measurement signal corresponding to a physical parameter. For example, multiple preparation modules are required if multiple image data sets from different positions of the patient are acquired and a separate preparation is required for every single one of these image data sets (known as slices).
In EKG-triggered or breathing-triggered data acquisition, a partial measurement is started after detection of the trigger signal. One or more data acquisitions can ensue in this partial measurement. These data acquisitions can belong to a single image data set, or multiple data acquisitions can belong to multiple image data sets, or multiple data acquisitions can respectively belong to a single image data set. The design and the sequence of corresponding measurements are hereby known and therefore require no detailed explanation herein.
The time span provided between two R-spikes is designated as an RR-interval. The partial measurement to be conducted in this interval includes not only the data acquisition as such but also, as described, preparation steps necessary for this in the form of magnetization preparation modules. This is discussed below in further detail.
In a known method for EKG-triggered implementation of a magnetic resonance measurement, a partial measurement is conducted after detection of an R-spike. After the end of the partial measurement, the control device of the magnetic resonance tomography apparatus communicates with the EKG device in order to be able to start the next partial measurement after displaying the next R-spike. Since the heart beat of a patient is subject to a certain variability, a safety interval from the end of the (last) partial measurement to the point in time of the expected next R-spike is typically observed (maintained) in order to be able to compensate for such fluctuations. The full RR-interval is accordingly not utilized; the RR interval beginning with this R-spike is not used to implement a partial measurement.